The present invention relates to an antenna.
A conventional antenna will be described referring to FIGS. 33 to 36. As well shown in FIG. 33, the antenna 130 comprises a chassis is configured with a grounding conductor 131 provided as the bottom surface thereof, two top conductors 135 and 118 provided as the top surface thereof opposite to the grounding conductor 131, and side conductors 134 provided as the antenna sides. The grounding conductor 131, the side conductors 134, and the ceiling conductors 135 and 138 are electrically connected to each other. A feeding point 132 is provided on the grounding conductor 131 for receiving electric power from the outside. The feeding point 132 is electrically connected to one end of an antenna element 133 made of a conductive wire while the other end is connected electrically and mechanically by soldering or the like to a linear conductor 139 which is provided at the center on the top surface of the antenna. Furthermore, there is a pair of openings 136 and 137 provided symmetrically on both sides of the linear conductor 139 on the top surface of the antenna for radiation of electric waves.
FIG. 34 illustrates an example of setting dimensions of the antenna 130. It is assumed in FIGS. 33 and 34 that the X, Y, and Z set a three-dimensional coordinate space. The antenna 130 is arranged with the grounding conductor 131 sitting on the XY-plane, the feeding point 132 defining the origin, and the linear conductor 139 extending along the Y-axis, hence having a symmetrical structure to each of the ZY-plane and the ZX-plane. In this example, the grounding conductor 131 is formed of a square shape having each side of 0.76xc3x97xcex along the X and Y-axes (xcex being the free space wavelength) based on the free space wavelength. The height along the Z-axis of the side conductors 134 is set as 0.08xc3x97xcex. The length along the X-axis of the openings 136 and 137 provided on both sides of the linear conductor 139 at the center of the top surface of the antenna is 0.19xc3x97xcex while the side along the X-axis of the ceiling conductors 135 and 138 is set as 0.19xc3x97xcex. The length along the Z-axis of the antenna element 133 is set as 0.08xc3x97xcex.
FIG. 35 illustrates a VSWR characteristic curve of the input impedance characteristic to a 50 xcexa9 feeding line in the antenna 110 set as described. The horizontal axis in the figure is normalized by the resonance frequency f0. It is then apparent from the figure that the frequency band lower than 2 of VSWR extends 10% or higher, and the reflection loss is smaller throughout the wide band resulting in improvement of the impedance.
FIG. 36 illustrates the radiation directivity on the antenna 130. The circular chart expressed the radiation directivity is 10 dB per scale and the unit is dBi based on the radiation power at the point waveform source. As apparent from the diagram, the antenna 130 has a bidirectivity of electric wave radiation along the X direction while along the Y direction is minimized. The antenna 130 having such characteristics is useful in a long, narrow interior space such as a corridor.
The antenna 130 has the openings 136 and 137 provided in the top surface thereof for radiation of electric waves. As the antenna element 133 acting as the electric wave radiation source is surrounded by the grounding conductor 131 and the side conductor 134, the electric wave radiation effect will be negligible to the four sides and the bottom (i.e. a positional environment). According to the above characteristic, the antenna 130 can simply be mounted to any indoor location such as a ceiling with the body embedded but the top surface exposed to the radiation space so that it is flush with the ceiling surface. As a result, the antenna exhibits the projecting object from the setting surface thus being less noticeable in the view and more preferable in the appearance.
Also, in the antenna 130, the height of the antenna element 133 is set as 0.08xc3x97xcex and it is lower than that of a known xc2xc wavelength antenna element. This contributes to the downsizing of the antenna. Accordingly, even if the antenna is hardly embedded in the setting surface such as a ceiling, the projecting object can be minimized thus being less noticeable in the view and more preferable in the appearance.
Moreover, the antenna 130 is symmetrical structure on both the ZY-plane and the ZX-plane. This permits the directivity of electric wave radiation to be symmetrical toward each of the ZY-plane and the ZX-plane.
However, the conventional antenna 130 having the foregoing structure can be resonant only at an odd number multiple of the fundamental frequency but hardly operated at any desired group of frequencies. It is hence necessary for radiation of electric waves at different frequencies to provide a corresponding number of the antennas. The more the number of the antennas, the greater the space for installation of the antennas will be increased. Also, an increase in the number of the antennas requires a more number of transmission lines thus further increasing the installation space. Accordingly, when the installation space is too large, the antenna can hardly be mounted with less visibility thus failing to improve the appearance.
The present invention has been developed in view of the above technical drawbacks and the object is to provide an antenna which can radiate electric waves at a plurality of desired frequencies while it is made relatively simple in the structure and minimized the antenna body.
In an aspect of the present invention, there is provided an antenna comprising: a chassis consisting mainly of a grounding conductor provided as a bottom surface, a ceiling conductor provided as a top surface opposite to the grounding conductor, and side conductors provided as antenna sides; at least one opening provided in apart of said chassis, which opens for radiation of electric waves; a feeding point provided on said grounding conductor for power supply via a predetermined feeding line from the outside; and an antenna element connected to said feeding point at one end while being connected to said ceiling conductor via a frequency selectable circuit at the other end, and surrounded by the side conductors.
Said ceiling conductor may have a generally annular slit provided therein about the joint between said antenna element and the ceiling conductor, and the inner edge and the outer edge forming the slit of the ceiling conductor may be connected to each other via a frequency selectable circuit different from the frequency selectable circuit at said joint between said antenna element and the ceiling conductor.
Two or more of said generally annular slits may be provided concentrically, and the outer edge and the inner edge forming each of the slits of the ceiling conductor may be connected to each other via respective frequency selectable circuits.
Said chassis may be situated in an XYZ orthogonal coordinate system with said grounding conductor extending along the XY-plane and said feeding point sitting at the origin so that said grounding conductor, the ceiling conductor, and the side conductors are symmetrical about the ZY-plane and the opening in said chassis is symmetrical about the ZY-plane.
Said chassis may be situated in an XYZ orthogonal coordinate system so that said grounding conductor, the ceiling conductor, and the side conductors are symmetrical about the ZX-plane and the opening in said chassis is symmetrical about the ZX-plane.
Said frequency selectable circuit may be configured with a parallel resonance circuit.
Said frequency selectable circuit may be configured with a low-pass filter.
Said frequency selectable circuit may be configured with a changeover switch.
Further, said antenna may comprise a matching conductor provided to match the impedance with said feeding line and electrically connected to the grounding conductor. Said matching conductor may be coupled via the frequency selectable circuit to the grounding conductor. Said matching conductor may be electrically connected to the antenna element.
The inner space of said chassis may be filled partially or entirely with a dielectric.
Said ceiling conductor may be a pattern of a metallic material provided on the dielectric substrate.
Further, said antenna may comprise an electric field adjusting conductor for changing a distribution of the electric field across said opening.
Said electric field adjusting conductor may be coupled via the frequency selectable circuit to said chassis.
Further, said antenna may comprise an opening space variable means for changing the opening space of the opening provided on said chassis.
The grounding conductor provided as the bottom surface of the antenna may be arranged of a circular shape.
Further, said antenna may comprise a transmission/reception circuit for transmitting and receiving signals of a specific frequency or frequency band, said transmission/reception circuit being connected at one end to said antenna element while being connected at the other end to a signal transmission cable which communicates with a predetermined device for processing a baseband signal.
Said transmission/reception circuit may be accommodated in the chassis and shielded with a cover member.
Said grounding conductor may have a hollow protrusive portion provided thereon and the transmission/reception circuit may be located on the lower side of the grounding conductor so as to be accommodated in the hollow space of the protrusive portion.
Said hollow space of the protrusive portion of said grounding conductor may be shielded with a cover member that is provided on the lower side of the grounding conductor.
Said transmission/reception circuit may be composed of passive elements without a power supply.
Said transmission/reception circuit may include a high frequency IC capable of controlling the frequency or frequency band of a signal to be received or transmitted.
Said transmission/reception circuit may include a filter having a predetermined passing frequency band.
Said transmission/reception circuit may include a filter switching circuit having a plurality of filters which are different from each other in the passing frequency band and a filter switch for switching between the filters so that one of the filters becomes available.
Said transmission/reception circuit may include an amplifier for transmission and/or an amplifier for reception.
Said transmission/reception circuit may include a plurality of amplifiers which are different from each other in the gain for transmission and/or reception.
A plurality of said amplifiers for transmission may be connected to said signal transmission cable via a signal divider, said signal divider dividing a signal input from said signal transmission cable to a plurality of signals and outputting the signals to said amplifiers for transmission.
A plurality of said amplifiers for reception may be connected to said signal transmission cable via a signal compositor, said signal compositor compounding a plurality of signals input from said amplifiers for reception to one signal and outputting the signals to said signal transmission cable.
Said signal transmission cable may be an optical fiber, and said transmission/reception circuit may include a light passive element for transmission capable of photoelectric conversion and/or a light active element for reception capable of electric-optic conversion, each of which is connected to said optical fiber.
Said optical fibers to which said light passive element or said light active element is connected, may be coupled to one optical fiber via a photocoupler.